Electrode material comprising graphene-composite materials in a graphite network

ABSTRACT

A durable electrode material suitable for use in Li ion batteries is provided. The material is comprised of a continuous network of graphite regions integrated with, and in good electrical contact with a composite comprising graphene sheets and an electrically active material, such as silicon, wherein the electrically active material is dispersed between, and supported by, the graphene sheets.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/940,241 that was filed Nov. 5, 2010, which claims priorityfrom U.S. provisional patent application No. 61/258,801 that was filedNov. 6, 2009, the entire contents of which are hereby incorporated byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under DE-FG02-01ER15184awarded by Department of Energy. The government has certain rights inthe invention.

BACKGROUND

Rechargeable Li ion batteries have a wide range of applications. Theyare used to supply electricity to many portable electronic devices andhand-held tools, such as laptop computers, cell phones and otherwireless communication devices, cordless electrical tools, and others.They can also be used in automobiles, trucks, airplanes, and othermobile devices either as the primary or sole power source, or as anauxiliary power source. An example is their use in hybrid vehicles andelectric vehicles. Rechargeable batteries also can be used as a deviceto store electricity generated from intermittent sources, such as windturbines or solar panels.

The performance of the Li ion battery is important in any of theseapplications. In general, the performance is measured by the chargedensity or storage capacity (how much electric charge can be stored perunit weight or volume), power density (rate of discharge per unit weightor volume), cycling durability (the number of charge-discharge cyclesthat can be repeated while maintaining the storage capacity and powerdelivery capability), and safety. The first three, namely storagecapacity, power density, and cycling durability are determined primarilyby the electrically active components of the battery.

The storage capacity, power density, and cycling stability dependstrongly on the nature of the electrically active material (EA) and howit is supported and electrically connected to the current collector,which transfers electrons between the EA and the outside world. In atypical commercial Li ion battery, the negative electrode uses graphitepowder as the EA, which is bonded together and to a metallic currentcollector with a binder. The maximum storage capacity of the graphite isdetermined by the chemical stoichiometry as one Li per six carbon atoms,giving a charge density of about 380 mAh/g of graphite. The storagecapacity can be increased significantly using other EAs, such as Si, Sn,and many other elements as well as bimetallic or multimetallic mixtures.A major obstacle to the use of these alternative EAs is cyclingstability. For example, the theoretical storage capacity of Si is about10 times higher than graphite, but for negative electrodes made ofsilicon nanoparticles (e.g., particles of tens of nm diameter), theinitial high capacity is lost after a few cycles to less than 10% of thetheoretical capacity.

Silicon is often used as an example of an electrically active material,being an attractive candidate because it possesses the highesttheoretical energy density among common elements, is cheap, and easy tohandle. Various forms of Si electrode materials have been tested,including Si particles mixed with a binder and conducting carbon,nanowires, thin films, and 3-dimensional porous particles. (See, forexample, B. A. Boukamp, G. C. Lesh and R. A. Huggins, J. Electrochem.Soc., 1981, 128, 725-729; B. Gao, S. Sinha, L. Fleming and O. Zhou,Advanced Materials, 2001, 13, 816-819; J.-K. Lee, M. C. Kung, L. Trahey,M. N. Missaghi and H. H. Kung, Chem. Mater., 2009, 21, 6-8; C. K. Chan,H. Peng, G. Liu, K. McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui,Nat. Nanotechnol., 2008, 3, 31-35; T. Takamura, M. Uehara, J. Suzuki, K.Sekine and K. Tamura, J. Power Sources, 2006, 158, 1401-1404; and H.Kim, B. Han, J. Choo and J. Cho, Angew. Chem., Int. Ed., 2008, 47,10151-10154, S10151/10151-S10151/10153). However, these are still notsatisfactory, either because of poor cycling stability, cost ofmanufacturing, and/or insufficient capacity improvement. Although theexact causes for storage capacity loss upon cycling are still underinvestigation, one contribution is fracturing of the Si structureconsequent to the large volume changes upon lithiation/delithiation,resulting in loss of electrical contact of some Si fragments. Variousattempts to stabilize these structures have been reported. The mostcommon approach is to encapsulate the Si structure with a conductingcarbonaceous layer, in hope that this would better retain the Sifragments from being disconnected from the conducting electrode. Variousprecursors can be used for encapsulation, includingresorcinol-formaldehyde gel, poly(vinyl chloride)-co-vinyl acetate orpolyvinyl chloride and chlorinated polyethylene, glucose, and fullereneC₆₀. (See, for example, J. K. Lee, M. C. Kung, L. Trahey, M. N. Missaghiand H. H. Kung, Chem. Mater., 2009, 21, 6-8; Y. Liu, Z. Y. Wen, X. Y.Wang, X. L. Yang, A. Hirano, N. Imanishi and Y. Takeda, J. PowerSources, 2009, 189, 480-484; Q. Si, K. Hanai, N. Imanishi, M. Kubo, A.Hirano, Y. Takeda and O. Yamamoto, J. Power Sources, 2009, 189, 761-765;Y. S. Hu, R. Demir-Cakan, M. M. Titirici, J. O. Muller, R. Schlogl, M.Antonietti and J. Maier, Angewandte Chemie-International Edition, 2008,47, 1645-1649; and A. A. Arie, J. O. Song and J. K. Lee, Mater. Chem.Phys., 2009, 113, 249-254). Noticeable improvements were achieved, butcapacity degradation was not eliminated.

In many of the engineered structures examined, such as nanowires andthin films, Si and other high capacity materials exhibit storagecapacities that are near the theoretical value. However, the need tomaintain electric conductivity with the current collector limits theirdimensions to hundreds of nanometers. Furthermore, these structurestypically require a metallic current collector as support, the weight ofwhich significantly lowers the overall electrode storage capacity of theelectrode assembly.

BRIEF SUMMARY

Electrode materials, electrodes made from the materials, batteriesincorporating the electrodes, and methods for making the electrodematerials are provided.

In the present electrodes, high capacity EA particles (or EA thin films)can be finely dispersed in (deposited on) a light-weight, electricallyconducting, mechanically sturdy matrix that is sufficiently flexible toaccommodate volume changes of the EA during use, and which can form aself-supporting electrode structure without the need for binders oradditional current conductors that add weight to the electrode whilecontributing minimally to the storage capacity.

Electrodes that exhibit high storage capacities and good cyclingstability can be prepared starting with graphene sheets, derived fromlow cost graphite and using a simple, easily scalable procedure in whichan electrically active material, in the form of nanoparticles and/orthin films, are dispersed in, or deposited on, a graphene composite, anda portion of the graphene sheets is subsequently reconstituted intographite to form a continuous, highly conducting network that alsoserves as a structural scaffold to anchor the graphene sheets thatsandwich and trap the active material nanoparticles and/or thin films.

A schematic of a structure of one embodiment of an electrode material isshown in FIG. 6. This structure includes a graphite network and acomposite comprising particles of an electrically active materialdispersed between graphene sheets, wherein the composite is integratedinto the graphite network to form an integral structure. As shown in thefigure, the graphite network can be a continuous 3-dimensional networkof graphite regions 602, supporting and connected to the compositematerial 604, where the graphitic regions (i.e., the regions comprisingthe reconstituted graphite) are characterized by ordered layers ofcarbon and the composite regions are characterized by disordered(non-crystalline) layers of carbon (graphene). Suitable electricallyactive materials include electrically active nanoparticles (e.g.,particles having an average size of no greater than about 100 nm), suchas nanoparticles of silicon (Si), tin (Sn), germanium (Ge), gallium(Ga), their alloys and intermetallics, and lithium titanium oxide. Thenanoparticles can have a variety of shapes, including, but not limitedto, spherical, elliptical, rod-like, tube-like, rectangular, cubic, orirregular.

In other embodiments, the electrically active material is present in theelectrode materials as thin, continuous or discontinuous, coatings(films) on the graphene in the composite region, the coatings having athickness of, for example, 5 nm or less, 2 nm or less, or even 1 nm orless. Such thin coatings of the electrically active material can be usedinstead of, or in addition to, the nanoparticles of electrically activematerial.

Thus, one aspect of the invention provides an electrode materialcomprising a continuous network of crystalline graphite regionsintegrated with, and in electrical contact with, a composite comprisingdisordered graphene sheets and nanoparticles (or thin films) comprisingan electrically active material, wherein the nanoparticles (or thinfilms) comprising the electrically active material are dispersed between(or coated on) the disordered graphene sheets.

Another aspect of the invention provides a lithium ion batterycomprising a cathode, an anode and a non-aqueous electrolyte disposedbetween the cathode and the anode, wherein the anode comprises anelectrode material comprising a continuous network of graphite regionsintegrated with, and in electrical contact with, a composite comprisingdisordered graphene sheets and nanoparticles (or thin films) comprisingan electrically active material, wherein the nanoparticles (or thinfilms) comprising the electrically active material are dispersed between(or coated on) the disordered graphene sheets.

In some embodiments, the anode of the lithium ion battery has a lithiumion storage capacity of at least 1000 mAh/g, or even 2000 mAh/g, andexperiences a decrease in lithium ion storage capacity of no greaterthan 1% per cycle over 50 lithiation/delithiation cycles.

Yet another aspect of the invention provides a method for making anelectrode material, the method comprising: (a) mixing nanoparticlescomprising an oxidized electrically active material with oxidizedgraphene sheets to form a silicon-graphene oxide composite materialcomprising the nanoparticles dispersed between disordered graphenesheets; and (b) thermally reducing the silicon-graphene oxide compositematerial such that some of the disordered graphene sheets form regionsof crystalline graphite, thereby forming an electrode materialcomprising a continuous network of graphite regions integrated with, andin electrical contact with, a composite comprising disordered graphenesheets and nanoparticles comprising an electrically active material,wherein the nanoparticles comprising the electrically active materialare dispersed between the disordered graphene sheets.

The step of mixing the nanoparticles comprising an oxidized electricallyactive material with oxidized graphene sheets to form thesilicon-graphene oxide composite material can be carried out by mixingan aqueous dispersion of the oxidized nanoparticles with an aqueousdispersion of the oxidized graphene sheets, sonicating the resultingmixture to form the silicon-graphene oxide composite material, andfiltering off the water and allowing the silicon-graphene oxidecomposite material to dry.

In a variation of this aspect of the invention, the nanoparticles instep (a) can be replaced by (or augmented by) thin films of anelectrically active material. Such thin films can be applied to thegraphene sheets using, for example, chemical vapor deposition,plasma-enhanced chemical vapor deposition or magnetron sputtering. Thisvariation of the method provides an electrode material comprising acontinuous network of graphite regions integrated with, and inelectrical contact with, a composite comprising disordered graphenesheets and thin films comprising an electrically active material,wherein the thin films comprising the electrically active material aredisposed the disordered graphene sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. AFM scan of two graphene oxide sheets, showing a single layerheight of about 1 nm and a width of about 1×1 μm.

FIG. 2. X-ray diffraction patterns of: (a) graphene oxide paper; (b)silicon-graphene oxide (SGO) paper; (c) graphene paper formed by thereduction of graphene oxide (GO) paper; (d) silicon-graphene (SG) paperformed by the reduction of SGO paper. Diffractions due to graphite(26.4°) and Si (111), (220), and (311) at 28.3°, 47.2°, and 56.1°,respectively, are indicated.

FIG. 3. Edge-view SEM images of: (a) SGO paper; (b) and (c) SG paper.TEM images of: (d) Graphene sheets, and (e) SG paper. FIG. 3(f) shows areconstituted graphite phase composed of about 5-13 layers of graphene.FIG. 3(g) is an image of a crystalline Si nanoparticle, showing itsatomic lattice. FIG. 3(h) is an edge view of a crushed SG compositefragment showing the layered structure of graphene and Si nanoparticles.

FIG. 4. Delithiation capacities and coulomb efficiencies of SG samplesreduced at 550 or 850° C., using a constant current-constant voltage(CCCV) method (1.5-0.020V, 1000-50 mA/g).

FIG. 5. Delithiation capacity of: (a) SG paper (sample 2 from theExamples), 61 wt. % Si; shown with coulombic efficiencies; (b) SG paper(sample 1 from the Examples), 59 wt. % Si; tested using the CCCV method(sample 1 from the Examples: 1.5-0.005 V, 1000-80 mA/g; sample 2 fromthe Examples: 1.5-0.02 V, 1000-50 mA/g); (c) SG paper (sample 1 from theExamples), crushed and mixed with a poly(vinylidene fluoride) (PVDF)binder, cycled at 100 mA/g constant current mode (2.0-0.02V); (d)Graphene-only sample cycled using the CCCV method.

FIG. 6. Schematic drawing (not to scale) of a high-capacity, stableelectrode, made of a 3-D network of graphite (lighter lines) anchoringregions of graphene-Si composite. Circles denote Si nanoparticles anddarker lines graphene sheets. The graphite network is formed fromregions of graphene sheet stacks; thus, the graphite and the grapheneregions have similar number of sheets.

FIG. 7 shows XRD spectra of SG composites reduced at 550° C. (top), 700°C. (middle), and 850° C. (bottom).

DETAILED DESCRIPTION

A durable electrode material suitable for use in Li ion batteries isprovided. Methods of forming the electrode material are also provided.The material is comprised of a network of graphite regions that are ingood electrical and physical contact with composite regions comprising acomposite of graphene sheets and nanoparticles of an EA and/or thinfilms of an EA, wherein the EA nanoparticles and/or EA thin films aredispersed in, and supported by, the graphene sheets. The network ofgraphite regions is formed by reconstituting graphene sheets at thecontact points of stacks of graphene sheets after the EA nanoparticles(or EA films) have been dispersed with (or deposited on) the graphenesheets. During the reconstitution process, highly dispersed EAnanoparticles (or EA films) are trapped between the graphene sheetswithin the composite regions of the material.

Unlike previous composites of graphene sheets and EA materials, thepresent materials provide a continuous network of graphite regions,formed from reconstituted graphene stacks in a graphene sheet-EAcomposite. Such a structure offers the combined advantages of the highelectrical conductivity and the structural stability of graphite withthe flexibility of graphene sheets, and the high Li ion storage capacityof the EA material. As shown for the embodiment depicted in FIG. 6 andFIG. 3, the network of graphite is continuous because continuous sheetsof graphene extend through the graphite regions and the graphene-EAnanoparticle composite to form an integral structure. Such a structureis distinguishable from a non-continuous structure wherein EAnanoparticles are located within pores between graphite flakes, graphiteworms or graphite platelets.

The graphite regions that are made from stacks of graphene sheets, areelectrically conducting, mechanically strong, and easy to prepare fromexfoliated graphite. They possess limited Li storage capacities,consistent with carbon-based materials. Since they can be made from alow cost starting material and the preparation process is inexpensiveand readily scalable, they are well-suited as supports for high-storagecapacity materials.

In the graphene sheet-EA composite regions, the flexibility of thegraphene sheets makes it much easier for the material to accommodatevolume changes of the EA material during the charging and dischargingcycle. The extremely high surface area of the graphene sheets provides alarge number of contact points with the EA material and, thus, theability to maintain electrical contact even when the EA materialundergoes morphological changes and/or agglomeration or fracturingduring cycling, thus effectively overcoming one of the principal causesof performance degradation (capacity fading). The network of graphiteregions in the present materials provides an excellent electricalconductivity path and a structurally robust scaffold to support thegraphene-EA composite. By forming the graphite network directly fromgraphene-EA composites at points where graphene sheets are in contactwith each other, such that the network of graphite regions and thegraphene-EA composite forms one integral piece, the high electricalconductivity across the graphene-graphite interface and the structuralsupport of the graphite network are ensured.

Any suitable EA materials for the negative and positive electrode can beused, such as Si, Sn, or other monometallic, bimetallic, ormultimetallic materials, or oxidic or sulfide materials, or theirmixtures. If silicon is chosen as the electrically active material,typical loadings for the silicon in the electrode materials are in therange of about 30 wt. % to about 80 wt. %. For example, in someembodiments the electrode materials comprise about 50 wt. % to about 70wt. % silicon. This includes embodiments in which the electrodematerials comprise about 55 to about 65 wt. % silicon.

Electrodes made from the present materials are typically no greater than50 μm thick. This includes embodiments of the electrodes that are nogreater than 15 μm thick and further includes embodiments of theelectrodes that are no greater than 5 μm thick.

Electrodes made with the present electrode materials can provide alithium ion storage capacity of at least 1000 mAh/g. This includesembodiments of the electrodes that provide storage capacities of atleast 1500 mAh/g and further includes embodiments of the electrodes thatprovide storage capacities of at least 2000 mAh/g. In addition, theelectrodes can provide very good cycling stability. Thus, someembodiments of the electrodes experience a decrease in lithium ionstorage capacity of no more than 1% per cycle over 50 cycles. Thisincludes embodiments of the electrodes that experience a decrease inlithium ion storage capacity of no more than 0.5% per cycle over 50cycles (or even over 200 cycles) and further includes embodiments of theelectrodes that experience a decrease in lithium ion storage capacity ofno more than 0.2% per cycle over 50 cycles.

An additional advantage of the present materials is the low cost ofgraphene sheets and the ease of manufacturing of the graphene sheet-EAcomposite-graphite network assembly. A third component can be addedadvantageously to the EA-graphene composite. Such third component caninclude a polymer or a carbonized form of polymer that can assist indispersing the EA material and stabilizing against agglomeration orother chemical degradation.

Li ion batteries incorporating the present electrode materials are alsoprovided. The batteries include a cathode, an anode and a non-aqueouselectrolyte disposed between the cathode and the anode. The anodecomprises the above-described electrode material comprised of a networkof graphite regions that are in good electrical and physical contactwith a composite of graphene sheets and an EA material supported by thegraphene sheets.

EXAMPLE

This example demonstrates stable performance in maintaining storagecapacity against fading with cycling of graphitenetwork/graphene-silicon nanoparticles composites for the Li ionnegative electrode. This example describes composites of Sinanoparticles highly dispersed between graphene sheets, and supported bya 3-D network of graphite regions formed by reconstituting regions ofgraphene stacks. An electrode with a storage capacity higher than 2200mAh/g that decreased by 0.16% or less per cycle after 50 cycles, and by0.5% or less per cycle after 200 cycles, was prepared.

Preparation of Graphite Oxide:

Graphite oxide (GO) was synthesized from flake graphite (Asbury Carbons,230U Grade, High Carbon Natural Graphite 99+) by a modified Hummersmethod originally reported by Kovtyukhova et al. (N. I. Kovtyukhova, P.J. Ollivier, B. R. Martin, T. E. Mallouk, S. A. Chizhik, E. V. Buzanevaand A. D. Gorchinskiy, Chem. Mater., 1999, 11, 771-778), in whichpre-oxidation of graphite is followed by oxidation with Hummers' method.In the pre-oxidation, the graphite power (2.0 g) was added with stirringinto a concentrated H₂SO₄ (20 mL) solution in which K₂S₂O₈ (1.0 g) andP₂O₅ (1.0 g) were completely dissolved at 80° C. The mixture, in abeaker, was kept at 80° C. for 4.5 h using an oil bath, after which themixture was cooled down and diluted with 1 L of distilled deionized(DDI) water. The pretreated product was filtered with a nylon membranefilter (0.2 μm, Milipore) and washed on the filter until the pH of thefiltrate water became neutral. The shiny, dark-gray, pre-oxidizedgraphite was dried in air overnight. Then it was dispersed by stirringinto chilled H₂SO₄ (75 mL) in an Erlenmeyer flask in an ice bath. KMnO₄(10 g) was added slowly with stirring to keep the temperature ofreaction mixture below 20° C. The resulting thick, dark green paste wasallowed to react at 35° C. for 2 h followed by addition of DDI water(160 mL) to give a dark brown solution. To avoid over-flow of themixture due to rapid temperature rise with foaming by water addition,the flask was chilled in an ice bath and water was added in ˜5 mLaliquots with close monitoring of the temperature (kept below 50° C.).After additional stirring for 2 h, the dark brownish solution wasfurther diluted with distilled water (500 mL) after which H₂O₂ (30%, 8.3mL) was added slowly and the color of the mixture turned into abrilliant yellow. The mixture was allowed to settle overnight and thesupernatant was decanted. The remaining product was washed with 10% HClsolution (800 mL) with stirring and the brownish solution was allowed tosettle overnight. The supernatant was decanted and the remaining productwas centrifuged and washed with DDI water. The washing process wasrepeated until the pH of the solution became neutral (at this stage, theMn concentration in the supernatant was below 0.1 ppm by atomicabsorption spectroscopy (AAS)). The obtained product was diluted to makea ˜0.7% w/w aqueous dispersion for storage.

Preparation of Silicon-Graphene Paper Composites:

In a typical preparation, Si nanoparticles (H-terminated, <30 nm,Meliorum Nanotechnology, stored in Ar) were removed from an argonglovebox and exposed to air overnight to ensure that a hydrophilic oxidelayer was formed on the surface of the nanoparticles. Then the Sinanoparticles were weighed and dispersed in ˜1 ml DDI water bysonication for 15 minutes. A desired amount of the graphite oxidedispersion was then added to the suspension of Si nanoparticles. In somepreparations, the pH was adjusted to ˜10 with NH₄OH (29% in H₂O, FischerScientific), whereas in others, no pH adjustment was made. No cleartrend on the effect of pH adjustment could be identified. The compositemixture was then sonicated for 60 minutes and vacuum-filtered (47 mm,0.22 μm pore nylon filters, Whatman) until the surface of the compositeappeared dry. The resulting material is referred to as asilicon-graphene oxide (SGO) composite paper. After filtering, the SGOcomposite paper was removed from the nylon filter carefully with forcepsand allowed to air-dry for approximately 24 hours. Once dried, a razorblade was used to cut the SGO composite paper into smaller ribbons toallow them to fit into a quartz tube for thermal reduction. Samples forelectrodes were cut from these ribbons either before or after thermalreduction, although the material was easier to cut prior to thermalreduction. SGO composite papers were reduced with a 10% H₂ in argon flow(˜100 ml/min total rate) at 700° C. for 1 hour. Other reductiontemperatures (e.g., 550 and 850° C.) and times could be used.Afterwards, the samples were ready for testing.

The SGO composite paper changed from amber and somewhat translucent togrey and opaque after reduction to form a “silicon-graphene paper” (or“silicon-graphene composite paper”). This silicon-graphene paper (or SGpaper) is the material comprising a network of graphite regions that arein good electrical and physical contact with composite region comprisinga composite of graphene sheets and nanoparticles of an EA, wherein theEA nanoparticles are dispersed in, and supported by, the graphenesheets. This material was more brittle than the SGO material, but wasstill flexible and could be cut with a metal cutter into circular disksfor electrochemical testing. The weight % (wt. %) of Si was determinedby weight loss due to combustion of the graphene in a thermogravimetricanalysis apparatus. In some cases, the SG composite paper was crushedand mixed with a binder to form an electrode.

Sample Characterization:

X-ray diffraction (Rigaku X-ray diffractometer miniflex) measurementswere carried out with Cu K_(α) radiation using an operating voltage of40 kV, a step size of 0.01°, and a step time of 1 s. High-resolutiontransmission electron microscopy (HRTEM) was performed on a HitachiHF-2000 operated at 200 kV with a energy dispersive spectroscopy (EDS)detector. Samples dispersed in ethanol were applied onto a 400 mesh Cugrid with lacey carbon film and dried in air before TEM imaging. Forthermogravimetric analysis (TGA) (Mettler Toledo, TGA/SDTA851^(e)) inair, the samples were heated to 100° C. and held at 100° C. for 10 minto remove any volatiles and water adsorbed on the samples. Then, thetemperature was ramped to 800° C. at the rate of 10° C./min in flowingair. The weight loss, after correcting for oxidation of Si, was used tocalculate the carbon content. Separate TGA with bare siliconnanoparticle was run at the same conditions to obtain data to correctthe weight gain of Si-graphene composite papers due to oxidation ofsilicon particles in air. For atomic force microscopy (AFM) images ofgraphene oxide sheets, a droplet of the dilute, aqueous dispersion wasdried onto a freshly cleaved mica surface at room temperature with an N₂purge. A normal tapping mode silicon cantilever (125 kHz, 16 N/m,SI-DF20, Seiko Nano Instruments Inc.) was used for AFM imaging. An AFMinstrument (Nanoscope IV, Digital Instrument, Santa Barbara, USA) wasused for surface imaging. The dispersion and size distribution ofgraphene oxide sheets dispersed in water (0.25 mg/mL) were measured withdynamic light scattering (DLS) at 25° C. Scanning Electron Microscopy(SEM) images were taken with a Hitachi S-3400N-II microscope operated at5 kV accelerating voltage in the secondary electron (SE) mode. Sheetresistance and conductivity measurements were made with a four-pointprobe technique with an electrode separation of 1 mm using a Keithley2400 source meter.

The electrochemical response of the samples was tested usingtwo-electrode Swagelok-type cells, with a Li metal reference electrodethat was separate from a Li metal counter electrode. For the Si-graphenecomposite paper samples, the sample was weighed ten consecutive times onan analytical balance (Mettler Toledo AX205) to enhance accuracy. Thesesamples were typically ˜2 mg/cm² and 5-30 μm thick. They were placeddirectly on stainless steel plungers in the Swagelok-type cells. For thecrushed samples, the working electrode was prepared with mixtures of90.0 wt. % active materials and 10.0 wt. % poly(vinylidene fluoride)(PVDF, binder, Sigma-Aldrich) using a 5.0 wt. % PVDF solution inN-methyl-2-pyrollidone (NMP, 99.5%, anhydrous, Sigma-Aldrich) exclusiveof any conductive additive. The working electrode mixture was pastedonto a Cu foil (99.999%, 0.025 mm thick, Alfa-Aesar). Subsequently, thecoated electrodes were dried in a vacuum oven at 75° C. overnight. Thetypical active material loading of the electrode was ˜1-3 mg/cm² and itsthickness was ˜30-50 m. A 1.0M LiPF₆ in EC/DMC 1/1 (v/v) solution soakedon a microporous membrane (Celgard 2325) separator was used as theelectrolyte, and lithium foil (99.9%, 0.75 mm thick, Alfa-Aesar) wasused as the counter electrode. The cell was assembled in an argon-filledglove box.

Results:

A dried graphene oxide paper without Si nanoparticles showed a strong,broad x-ray diffraction peak at 6-10° 2θ (˜10 Å d-spacing). A similarintense peak at 8-10° 2θ was observed for a sample containing ˜60 wt. %Si (FIG. 2 curves a and b). These values are consistent with reportedinterlayer spacing for graphene. (See, for example, D. A. Dikin, S.Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko,S. T. Nguyen and R. S. Ruoff, Nature (London, U. K.), 2007, 448,457-460). For a SGO sample, peaks corresponding to (111), (220), and(311) diffractions of Si were also observed, suggesting that asprepared, the Si nanoparticles remained mostly in the metallic state.After reduction in H₂ to form a SG sample, the low-angle diffractionpeak of interlayer spacing disappeared, while a peak appeared at about26.4° on top of a broad hump, indicating the presence of bothcrystalline and disordered graphite phases (FIG. 2 curves c and d). TheSi diffractions were still clearly observed, and, from the line width ofthe (220) diffraction, the Si particles were estimated to have anaverage diameter of 21-22 nm, suggesting little particle coarsening.

The thermal reduction temperature and time significantly affected theextent of graphite reconstitution in SG samples (FIG. 7) and theirconductivity. The conductivities were 13.1, 18.7, 33.1 S/cm for SGsamples reduced at 550, 700, and 850° C., respectively, versus 6.8×10³¹S/cm for an unreduced SGO paper. The resistivities were 0.076, 0.053 and0.030 (Ω·cm) for SG samples reduced at 550, 700, and 850° C.,respectively, versus 146 (Ω·cm) for an unreduced SGO paper. Thecorresponding sheet resistances were 153, 107, 60, and 2.9×10⁵Ω for thethree SG samples and the SGO sample. The average size of thereconstituted graphite regions decreased as: 550>700>850° C. sample. Theextent of reconstitution in the present materials can vary based on theprocessing conditions and the loading of the electrically activematerial. However, in some embodiments, reconstituted graphite makes nomore than 50 percent (e.g., about 1 to 50 percent or about 5 to 40percent) by weight of the carbon in the materials.

The SEM image of a SGO composite paper (FIG. 3(a)) showed a stack ofgraphene oxide sheets. After reduction, the sheets appeared morecrumpled, and pockets of void space were clearly visible (FIGS. 3(b) and(c)). The physical thickness of the paper decreased upon reduction byabout 8% for a sample without Si, and about 5, 3.5, and 3% for samplescontaining 30-40 wt. %, 58-60 wt. %, and 78 wt. % Si. The sheet-likemorphology of graphene is also shown in the TEM images (FIG. 3(d)). FIG.3(e) shows Si nanoparticles, 20-25 nm in diameter, dispersed on/betweengraphene sheets in a SG sample. FIG. 3(f) shows a reconstituted graphitephase composed of about 5-13 layers of graphene. FIG. 3(g) is an imageof a crystalline Si nanoparticle, showing its atomic lattice. FIG. 3(h)is an edge view of a crushed SG composite fragment showing the layeredstructure of graphene and Si nanoparticles.

The effect of conductivity (and graphite crystallinity) on theelectrochemical behavior of the present materials was examined using twoSG samples reduced at 550 or 850° C. The results (FIG. 4) show that thesample with a higher conductivity retained better charge capacity uponcycling. This is consistent the hypothesis that a sample with a higherconductivity would be more tolerant to Si particles fracturing andredepositing on other parts of the graphene surface.

The Li ion storage capacities of two SG samples containing ˜60 wt. % Siare shown in FIG. 5. The Li ion storage capacities for the SG samplesare much higher than the storage capacity of graphene alone (curve d,FIG. 5). Curves a and b are for two samples that differ in theirthickness, being thinner (˜4 μm) for sample 2 (curve a) than sample 1(curve b, 15 μm). Sample 2 exhibited a very high storage capacity andstability, achieving >2200 mAh/g after 50 cycles and >1400 mAh/g after300 cycles with a Li ion storage capacity degredation of 0.16% or lessper cycle. Sample 1 exhibited a storage capacity of 1000 mAh/g after 100cycles, and retained a storage capacity of 800 mAh/g after 290 cycles,and 560 mAh/g after 600 cycles. Since the storage capacity of thesegraphene papers without Si was about 200 mAh/g, most of the observedcapacities were due to the Si nanoparticles. The cycling coulombefficiency was >95% throughout the test for sample 1. For sample 2, theefficiency increased from ˜80% initially to 98+% after ˜30 cycles.

The low storage capacities and coulomb efficiencies in the first fewcycles were probably due to the fact that the pores of the electrodewere not completely wetted with electrolyte initially, such that some ofthe Si nanoparticles in the center were not utilized.

Other methods to prepare SG composite electrodes are also available, butmay be less efficient. A number of samples were made by crushing a SGpaper into small chunks after reduction and forming an electrode using abinder. The resulting material showed initial storage capacities around1000 mAh/g and cycling stability similar to the paper composites, asillustrated in FIG. 5, curve d. The crushing process, mechanically brokedown the continuous nature of the SG paper and eliminated theadvantageous effects of physical confinement of the siliconnanoparticles and continuous connectivity of the 3-D network structure.

These results show that electrodes of high storage capacities and goodcycling stability can be obtained from graphene-Si composites providedthat the Si nanoparticles are well dispersed between the graphenesheets, and portions of the graphene sheet stacks are reconstituted toform a network of graphite regions within the material (FIG. 6). Thenetwork provides high electrical conductivity throughout the electrodeand serves as a mechanically strong framework to anchor the moreflexible graphene sheets in the graphene-EA composite that sandwich theSi nanoparticles. That the graphite is reconstituted from the graphenesheets ensures excellent electrical contact between the crystalline andnoncrystalline regions of the material, as well as mechanical integrityof the junctions between regions within the material.

As used herein, and unless otherwise specified, “a” or “an” means “oneor more.” All patents, applications, references, and publications citedherein are incorporated by reference in their entirety to the sameextent as if they were individually incorporated by reference.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art, all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeincludes the number recited and refers to ranges which can besubsequently broken down into subranges as discussed above. Finally, aswill be understood by one skilled in the art, a range includes eachindividual member.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein, but include modifiedforms of those embodiments including portions of the embodiments andcombinations of elements of different embodiments as come within thescope of the following claims.

What is claimed is:
 1. An electrode material, comprising: network ofgraphitic regions; and composite regions disposed between the graphiticregions, the composite regions comprising graphene sheets andelectrically active particles disposed between the graphene sheets,wherein at least some of the graphene sheets extend out of the compositeregions and are integrated into graphite of the graphitic regions. 2.The electrode material of claim 1 wherein the electrically activeparticles comprise a material selected from the group consisting ofsilicon, tin, an oxide, a sulfide, a metallic material, and mixturesthereof.
 3. The electrode material of claim 2 wherein the electrodematerial comprises between about 30 weight percent to about 80 weightpercent of the electrically active particles.
 4. The electrode materialof claim 1 wherein the graphitic regions comprise graphite that has beenreconstituted from graphene layers.
 5. The electrode material of claim 1wherein the electrode material has a thickness no greater than 50 μm. 6.An electrochemical cell, comprising: a first electrode comprising: anetwork of graphitic regions; composite regions disposed between thegraphitic regions, the composite regions comprising graphene sheets andelectrically active particles disposed between the graphene sheets; asecond electrode spaced from the first electrode; and an electrolyte,wherein at least some of the graphene sheets extend out of compositeregions and are integrated into graphite of the graphitic regions. 7.The electrochemical cell of claim 6 wherein the electrically activeparticles comprise a material selected from the group consisting ofsilicon, tin, an oxide, a sulfide, a metallic material, and mixturesthereof.
 8. The electrochemical cell of claim 6 wherein the electrodematerial comprises between about 30 weight percent to about 80 weightpercent of an electrically active material.
 9. The electrochemical cellof claim 6 wherein the second electrode comprises lithium.
 10. Theelectrochemical cell of claim 6 having a storage capacity of at least1,000 mAh/g.
 11. The electrochemical cell of claim 6 characterized asexhibiting a decrease in storage capacity no greater than 1% per cycleover 50 lithiation/delithiation cycles.
 12. The electrochemical cell ofclaim 6 characterized as exhibiting a decrease in storage capacity nogreater than 0.5% per cycle over 200 lithiation/delithiation cycles. 13.The electrode material of claim 1, wherein the network of graphiticregions comprises a three-dimensional network of electrically connectedgraphite.
 14. The electrochemical cell of claim 6, wherein the networkof graphitic regions comprises a three-dimensional network ofelectrically connected graphite.